1. Field of the Invention
This invention relates to metallization processes for use in making devices such as semiconductor devices, and to devices formed using those metallization processes.
2. Related Art
Formation of a metal layer is a common step in the formation of some devices, such as, for example, semiconductor devices. In particular, a metal layer can be formed so as to fill in vias or cover steps formed during fabrication of a semiconductor device. The formation of a metal layer over vias having a high aspect ratio (i.e., ratio of the depth of the via to the width or diameter of the via) or steps having a relatively large height has been subject to several problems, such as cusping and voiding.
In one previous method of forming a metal layer on a semiconductor wafer, the metal layer is formed using a two step process. In the first step, a relatively thick portion of the metal layer is deposited while the semiconductor wafer is held at a relatively cold temperature (i.e., preferably less than or equal to 200.degree. C.). The thickness of this portion must be adequate, in view of relevant process parameters (e.g., the geometry being metallized and the metal being used), to ensure that adequate metal is present to avoid the formation of voids during the metal reflow that occurs during the second step. For example, when the metal is an aluminum alloy, this thick portion preferably has a thickness equal to about 50% to 75% of the total thickness of the metal layer to be formed. Further, this portion is preferably deposited at a rate greater than about 150 .ANG./sec. In the second step, the remainder of the metal is deposited while the semiconductor wafer is held at a relatively high temperature (e.g., when the metal is an aluminum alloy, about 400.degree. C. to about 500.degree. C.) that allows the deposited metal to reflow through grain growth, recrystallization and bulk diffusion. The rate of deposition of the aluminum in the second step is preferably slower than that during the first step, but is preferably greater than about 50 .ANG./sec., and more preferably between about 100 .ANG./sec. and about 200 .ANG./sec. Further, the deposition rate can be increased during the second step to increase the process throughput. However, this method does not minimize the number of defects formed in the metal layer (such as result from cusping and/or voiding, for example) as much as desired.
In another previous method of forming a metal layer on a semiconductor wafer, the metal layer is also formed using a two step process including a first, cold deposition step followed by a second, hot deposition step. However, in this method, a relatively thin portion of the metal layer (e.g., 25% of the overall thickness) is deposited while the semiconductor wafer is held at the cold temperature, while a relatively large portion of the metal layer (e.g., 75% of the overall thickness) is deposited while the semiconductor wafer is held at the hot temperature. When the metal is an aluminum alloy, the wafer can be held at a temperature of about 200.degree. C. for a period of about 10 seconds during the cold deposition step. During the hot deposition step, a heated gas (typically argon) is flowed against the backside of the wafer to heat the wafer and the deposited metal. The wafer can be heated to a temperature of about 375.degree. C. to about 500.degree. C. For the illustrative temperatures given, the wafer is typically held at that temperature for about 3-5 minutes. However, the heated gas flow is kept relatively low (e.g., less than about 15 sccm and typically in the range between about 10 sccm and about 15 sccm) so that the pressure within the process chamber can be kept low (e.g., less than about 2 mtorr). Since the heated gas flow is kept relatively low, the wafer is not heated as fast as is desirable to minimize the number of defects formed (e.g., by cusping and/or voiding) in the metal layer. Increasing the temperature of the heated gas has been tried as a means to improve this method; however, the increased gas temperature causes the steady state temperature of the wafer during the hot deposition step to increase, thus increasing the likelihood of damaging the wafer (in particular, metallization that has been previously formed on the wafer). Causing the heated gas to impinge on the wafer at multiple locations has also been tried; however, while this can cause the distribution of defects to be more evenly spread throughout the metal layer, it does not adequately reduce the overall number of defects.